I've been playing since release (when i can) and have 80 Guardian and Mesmer, plus others in the works. . My Brother has expressed an interest in playing and will be soon. My question is this: Which toon combinations, really...for lack of a better word "gel" well with each other? Have not experienced other profession to there full extent...hence the question.
I understand ultimately its his choice what he plays and i will simply "fill the gap." True is i think any combination will work, but there HAS to be some that are more effective and fun combinations out there.

Here are some of my thoughts thus far:

Mesmer, sounds fun...Dual Phantasms build. Mucho bleed stacks. I also think confusion as a condition look much more viable and luring with two Shatter Mesmer via CoF.

Double Mesmer: bleed stacking from two phantasm mesmers can go up quite high without capping, which is good. Confusion stacks never have a problem of capping, so more the better. Having lots of illusions on the field have that "oomph", but one problem would be that you won't be able to tell apart your own illusions at times and hence it can make illusion management a bit harder. Other than that, the mesmer skills don't interfere with each other hence both of you can go with the same build and be twice as effective (chaining feedback/null field/time warp back to back). Alternatively you can go 1 phantasm damage build and 1 phantasm support build. In general, the double mesmer setup means that you're VERY vulnerable when there's tons of ground targeted aoe and/or other situations where both of your illusions do not live for long.

Double Guardian: with an emphasis on blind and aegis, and with both of you in there, you'd have very high blind and aegis uptime. Arguably however, for most PvE contents 1 precision sword guardian is all it takes to cover all the blind/aegis so some of your CDs may be wasted. Alternatively you can go 1 blind/aegis/shout guardian + 1 boon/consecration support guardian.

If he's going for single target damage, he should be going for either thief or mesmer. If he wants to do aoe damage, the melee professions, elementalist, as well as grenade engineer would be safe bet. Grenade engineer + arcana staff elementalist will demolish groups of mobs before they even touch you, stealth thief + stealth/shatter mesmer can burst down single targets fast, and it has that "fun" feeling, as well as making you absolutely hated in PvP. Dagger dagger elementalist/dagger dagger thief works nicely too with all the fast movement in combat.

Support/Damage Elementalist + Warrior/Thief/Ranger can max the benefit of combo fields and be quite effective, while support/damage elementalist + guardian can be nigh impossible to kill. Condition necro + iWarlock mesmer is another possibility since both profession hit harder with lots of different conditions on the enemy, and their conditions complement each other (mesmer supplying confusion while necro supplies the rest). Basically, when talking about synergy, heavy condition builds tend to overlap each other due to maxed bleed stacks/burn duration, and should be taken carefully. Heavy boon builds also have this problem, though less so, basically you don't want to stack boons to 3 minutes when the fight is only 30 seconds.

I heard a rumor that if you have one of each class, they go well together because they offer ALL the class advantages!

*mind blown*

Or you could just do any combo. Going Guardian/Warrior is the biggest nobrainer. Nothing can beat it unless we talk specific areas and hardcore builds (say WvW and two heartseeker stealthers vs single target).